Such a phenomenon that a certain substance emits visible light in response to various stimuli from the outside (external stimuli) has been known as so-called fluorescent phenomenon. A substance (fluorescent substance) exhibiting the fluorescent phenomenon is utilized in various fields such as a lamp, an illuminating lamp, a cathode-ray tube, various types of display device such as a plasma display panel, and a pigment. Further, a large number of substances (luminescent materials) emitting light in response to external stimuli such as ultraviolet light, electronic beam, X-ray, nuclear radiation, an electric field, or a chemical reaction have been known.
Meanwhile, the inventors of the present invention have found a stress-stimulated luminescent material that emits light due to strain caused by applying a mechanical external force, and have developed methods for evaluation thereof and utilization thereof.
Specifically, as the stress-stimulated luminescent material, the inventors of the present invention have developed a stress-stimulated luminescent body having a spinel structure, a corundum structure or a β-almina structure (see Patent Document 1), a silicate stress-stimulated luminescent body (see Patent Documents 2 and 3), and a high luminescence intensity stress-stimulated luminescent body made of defect-controlled aluminate (see Patent Document 4). The inventors of the present invention have also developed a method in which a stress distribution is visualized and evaluated by applying a mechanical force such as compression, tension, friction, and twisting, to a composite material including an epoxy resin and to a sample piece coated with a film made of the composite material (see Patent Documents 4 and 5). Also, the inventors of the present invention have developed a high luminescence intensity mechanoluminescense material having a structure in which a wurtzite structure and a zinc blende structure exist together and being mainly composed of oxide, sulfide, selenide, and telluride (see Patent Document 6).
Such a stress-stimulated luminescent body can repetitively emit light semi-permanently with such luminescence intensity that the emission can be confirmed by eyes. By using these stress-stimulated light emitting bodies, it becomes possible to measure a stress distribution in a structure including the stress-stimulated luminescent body. Patent Documents 7 and 8, for example, disclose such a method for measurement of the stress distribution.
[Patent Document 1]
    Japanese Unexamined Patent Application No. 2000-119647 (published date: Apr. 25, 2000)[Patent Document 2]    Japanese Unexamined Patent Application No. 2000-313878 (published date: Nov. 14, 2000)[Patent Document 3]    Japanese Unexamined Patent Application No. 2003-165973 (published date: Jun. 10, 2003)[Patent Document 4]    Japanese Unexamined Patent Application No. 2001-49251 (published date: Feb. 20, 2001)[Patent Document 5]    Japanese Unexamined Patent Application No. 2003-292949 (published date: Oct. 15, 2003)[Patent Document 6]    Japanese Unexamined Patent Application No. 2004-43656 (published date: Feb. 12, 2004)[Patent Document 7]    Japanese Unexamined Patent Application No. 2001-215157 (published date: Aug. 10, 2001)[Patent Document 8]    Japanese Unexamined Patent Application No. 2004-77396 (published date: Mar. 11, 2004)